Replacing alloy idler with steel

[Howard]

Hi wmg,

I can’t fault your reasoning, and I have to hold up my hands and say, it never occurred to me that of all the gear systems I’ve had apart, this is the only one with mesh adjustment.

What irritates me is when people (don’t take this personal, it’s not just in this case) apply theory and/or modern practices to a 60 year old engine. We’re not talking about an electric motor driving a gearbox with a constant load, we’re talking lumpy(ish) twin ic engine driving 2 camshafts with oscillating loads. I hate seeing 0.0008” talked about on the same page as a 60 year old engine. I’m willing to bet, that with the changing separation forces and the plain bearing clearances required, no large idler runs concentric, and if that’s being picky, I’m not the one talking about 10ths of a thou!

Phew! That’s that off my chest.

H

Ps I like the adjuster, it’s the only way to get my timing chest quiet enough for me to hear my straight through silencer.

 

[clevtrev]

I'm not questioning the distances, but what is your source, out of curiousity? The Works drawings I assume would specify it.

I should question the distances, Bruce. With the halftime pinion being 24 teeth and the cam pinion being 48 teeth, work it out for yourself.
The distance between crankshaft and large idler = 3.218"
The distance between Large Idler and Camshaft = 3.968"
This is arrived at by the sums of the diametric pitch. The diametric pitch of the HT pinion is 1.5" The cam being twice the size is obviously 3" The 79 tooth Idler is 4.9375" So you simply add the two together and divide by two.
As for a jig to set the Idler, forget it.

 

[bmetcalf]

You give me too much credit. It has been so long since I might have learned about tooth numbers and diametral pitch that I don't remember if I should have learned it or not. The recommended process of moving the spindle and selecting the right half time pinion allows me to escape the consequences of my scholastic shortcomings, thanks to PEI's work.

I should question the distances, Bruce. With the halftime pinion being 24 teeth and the cam pinion being 48 teeth, work it out for yourself.
The distance between crankshaft and large idler = 3.218"
The distance between Large Idler and Camshaft = 3.968"
This is arrived at by the sums of the diametric pitch. The diametric pitch of the HT pinion is 1.5" The cam being twice the size is obviously 3" The 79 tooth Idler is 4.9375" So you simply add the two together and divide by two.
As for a jig to set the Idler, forget it.

 

[Matty]

I am an electronic engineer with some qualifications in mechanical engineering and I used a very practical method to set up my Comet's and a friend's Rapide's new large steel idler.

There is of course more latitude with a single, because there is only one cam shaft to worry about and the large idler can in some cases be adjusted to mesh with both the camshaft pinion and several different half time pinions because they are not in line.

However, for a twin first adjust the large idler to mesh with virtually no backlash with both camshaft pinions - rotate the idler a few times to find any tight spots and set the idler adjusment to eliminate them. I found that the gears were not quite perfect on both my Comet and the twin I have just helped build, so this was necessary because the gears are not quite perfect .

Then choose a half time pinion which is a fairly tight fit with the large idler - try the old one first to get an idea of the best fit you probably need and order 3 different ones to bracket this size in order to be able to select the best one - check again for tight spots.

I believe with a fairly tight gear fit all should be well, because when the engine warms up, the alloy crankcase will expand more than the steel idler and give a good working clearance for the gears.

The bit which takes most of the time seemed to be selecting the shims to give the correct end float for the gears when the plate is bolted over them.

Both my Comet and the Rapide now have fairly quiet valve gear - for a Vincent anyway!!!

No measurements were carried out to achieve what I believe to be a good result.

Electrical engineers are guarenteed to give you a fit !!!

Matty